In the seventies we used to know which imposition scheme the printer was using on all our books. I can’t remember for sure, but this must have been important for paper purchasing. If the book was using “Imposition I” you’d need a different sheet size than if it was going to be “Imposition U”. To understand why we “allowed” the decision on the size of paper to be determined in the composing room goes back to the wildly different relationship of the various parts of our industry to one another in the age of letterpress and now.
Imposition is the arrangement of pages for printing in such a way that when the sheet is folded, the pages will be in the correct order and right way up. Sounds easy when you say it.
But just take a sheet of letter-sized paper and fold it in half across the short dimension first, then fold it in half again and again. Number the pages from the front to the back. When you open the paper, it will look like this. Note that the only two pages which appear next to one another are 8 and 9, which are the two pages at the center of this signature. If you made the first fold on the vertical dimension, you’d come up with a completely different picture. And this is only a 16-page signature. By changing the sequence of folds printers could achieve quite different, usable impositions.
Modern printing companies have standardized their manufacturing processes, and more and more folding is done on press. This means that many of the more exotic imposition schemes have been abandoned. In a sheet-fed letterpress world the folding department might be larger than the pressroom, and folding represented a significant bottleneck. When imposing pages you would also be dealing with pages of metal type: not nearly as easy to handle as pages of film or digital pages. The pages of metal type, each secured with a cord, would be laid out in position on a flat working-surface (the stone). A metal frame (the chase) would be placed round them, and the pages of type would be wedged against the inner edges of the chase with quoins. Chase, furniture and quoins were collectively called the forme (form in USA). This locked-up chase would be (carefully) moved to the press for printing. Each forme represented all the pages which were to print on one side of one sheet. This work was done in the composing room, and thus it was they who would decide on the imposition scheme. Of course they would consult with designer and pressroom, but I bet that the decision was often made on folding department capacity and loading, or even composing room preference. The publisher would then have to get in paper to fit.
When you were dealing with heavy metal formes, obviously changing anything could be expensive. The most economical printing job would be one which made an even working (UK) or even forms (USA). A book with exactly 128 pages will fold down to four 32 page sections with no wasted blank pages. A book of 132 pages will require you to deal with another section of 4 pages or 4 pages plus 4 blanks. This would have to be imposed for a different fold, and would be differentially costly, not only in folding but in the pressroom and the bindery too. When we moved to a world of imposing film negatives, the penalty for unlocking the forme disappeared. Not only that, but the “forme” wouldn’t be made up until the job had reached the printing department, not in composition. This means that there is no cost penalty involved in changing the page count of the book at the last minute — well no printing cost penalty — you will of course have to pay for the typesetting work involved. However on a book with a long run it will almost certainly cost you less to rerun it to fit the whole thing into that 128pp extent, rather than leaving it at 132 pages. The extra money paid to the typesetter will be less than your paper savings and the economies of printing and binding.
I don’t really know where the expression comes from, but if you have a PDF which is set up with double pages, page 2 next to page 3, 4 next to 5, and so on, just as it would look to you when you opened the book and looked at a spread, this tends to be referred to as being in “printer spreads”. If you have a PDF like that, you effectively have nothing, because the one thing you can say with absolute certainty is that (apart from the middle two pages of every sig) this is not how the printer needs the spreads to be set up. That decision won’t be made till the book is imposed, and of course what the printer needs from you is single pages, not any helpful combination of pages. Maybe the expression comes from the office printer: if you laid the book on the copier and copied two pages at a time, that’s what they’d look like.
It perhaps goes without saying, but there are too many examples for silence: all recto pages must have an odd page number, and all verso pages must have an even number. Once the book has started, it is unacceptable for any recto to be blank. (Recto from the Latin for right; verso from the Latin for turn over)